Anisotropic beam pumping of a Kerr lens modelocked laser

ABSTRACT

Apparatus and methods for anisotropic pumping of a Kerr lens modelocked laser. Direct diode laser pumping of an ultrafast Kerr lens modelocked laser oscillator is accomplished. Diode lasers generate severely anisotropic beams, meaning the pump beam has a higher-beam-quality dimension and a lower-beam-quality dimension. By spatially overlap of the pump beam higher-beam-quality dimension and the KLM laser mode, KLM operation is accomplished. Multiple laser diode pump beams are combined in counterpropagating and same-side configurations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to anisotropic beam pumping of a Kerr lensmodelocked laser. In particular, the present invention relates to usingdirect diode laser pumping of an ultrafast Kerr lens modelocked laseroscillator.

2. Description of Related Art

Ultrafast (i.e. pulsewidth of about 100 fs or less) lasers are used formany purposes, including seed sources for amplifiers and light sourcesfor scientific equipment. Laser oscillators can be thought of having twomain sub-modules: the pump source and the laser oscillator cavity. Thepump source serves as the optical power source for the oscillator; itconverts electrical power into optical power in a form that can be usedto generate the desired laser output. The purpose of the laseroscillator cavity is to convert the power from the pump source into thedesired optical characteristics for the given application.

Historically, Ti:sapphire has been the laser crystal of choice forultrafast laser oscillators because of its durability, wide bandwidth,pumping ease, and commercial availability. In order for the pump sourcelight to be converted to oscillator light, it must be absorbed by theTi:sapphire; FIG. 1 (Prior Art) shows this absorption curve 102, as wellas the fluorescence curve 104. As can be seen, the peak absorption isaround 500 nm but extends from about 450 nm to 600 nm.

Initially, the Argon Ion laser was used to pump Ti:sapphire because ofits emission near the absorption peak and excellent beam quality;however, the bulky, expensive, high-maintenance nature of this gas laserquickly gave way to solid state, frequency-doubled lasers such as Nd:YAGand Nd:YLF. Over the years, these solid state lasers have increased inreliability and power while decreasing in cost. However, these pumplasers all share the same complex architecture: laser diodes convertelectrical power to optical power; a laser cavity produces infra-redlight; and frequency-doubling crystal converts the infra-red light to awavelength which can be absorbed by Ti:Sapphire. Inherent in this pumplaser design was the assumption that the pump laser must have excellentbeam quality (quantified by the M² parameter being <˜1.5 (also called‘diffraction limited’)) in order to achieve Kerr Lens Modelocking (KLM),the process responsible for creating the ultrashort pulses. (See, forexample, Roth et al., “Directly diode-laser-pumped Ti:sapphire laser,”Opt Lett 34 3334-3336 (2009)).

Specific alternatives to the present invention include pumping ultrafastoscillators with anything that produces high beam quality in both axessuch as solid-state, frequency-doubled lasers (for example thoseproduced by Lighthouse Photonics), optically pumped semiconductor lasers(OPSELs), and gas lasers, such as those produced by Coherent Inc.

Indirect diode pumping of ultrafast oscillators using afrequency-doubled distributed Bragg reflector (DBR) tapered laser dioderequires complex and expensive components to create a high beam qualityoutput from a low beam quality laser diode source at a differentfundamental wavelength.

Direct-diode pumping of ultrafast oscillators has been demonstratedbefore, however, these implementations either required a saturablemedia, such as a saturable Bragg reflector (SBR), or were accomplishedwith a fiber-coupled diode laser with high beam quality in bothdimensions. The implementations that require a saturable Bragg reflector(SBR) for modelocking use the saturable reflection of the SBR ratherthan the nearly instantaneous Kerr Lens effect to implement modelocking.The advantages of Kerr Lens Modelocking over Saturable Absorbers andSaturable Reflectors are a reduction in component complexity, shorterfundamental pulse durations, and an increase in reliability. Saturablemedia are notorious for burning. The implementations of direct-diodepumping an ultrafast oscillator with a high beam quality in bothdimensions demonstrate Kerr Lens Modelocking (P. Wasylczyk, P. Wnuk, andC. Radzewicz, “Passively modelocked, diode-pumped Yb:KYW femtosecondoscillator with 1 GHz repetition rate,” Opt. Express 17, 5630-5635(2009).), but high beam quality fiber-coupled diodes of sufficientoutput power are not available at the correct wavelengths for pumpingmany gain media in which Kerr Lens Modelocking is advantageous,specifically Ti:sapphire. Therefore, this invention enables Kerr LensModelocking of ultrafast oscillators that use gain media where powerful,high beam quality diodes are not available for use as a pump laser, butwhere powerful diodes are available for use as a pump laser that havegood beam quality in one axis but not the other.

Other alternatives include Kerr Lens Mode locked Ti:sapphire lasers andSemiconductor Saturable Absorber Mirror (SESAM) mode locked Ti:Sapphirelasers.

A need remains in the art for a laser pump scheme using anisotropicpumping of a Kerr lens modelocked laser.

SUMMARY

It is an object of the present invention to provide a laser pump schemeusing anisotropic pumping of a Kerr lens modelocked laser. Inparticular, the present invention uses direct diode laser pumping of anultrafast Kerr lens modelocked laser oscillator, for example aTi:sapphire laser.

Thus, in embodiments of the present invention, the pump source consistsonly of laser diodes which convert the electrical power directly intooptical pump power at a wavelength which can be absorbed by Ti:Sapphire.These pump beams typically are anisotropic, meaning that they have ahigher beam quality in one dimension than in the other.

A direct laser diode pumped, Kerr lens modelocked, laser comprises adiode pump laser that generates an anisotropic pump beam having ahigher-beam-quality dimension and a lower-beam-quality dimension, anoscillator cavity including a gain medium, and optics for directlycoupling the pump beam into the gain medium without frequency doublingor the like. The various elements are constructed and arranged such thatwhen the laser is modelocked, spatial overlap of the pump beamhigher-beam-quality dimension and the KLM laser mode is improved, overspatial overlap of the pump beam higher-beam-quality dimension and thecontinuous wave mode. The gain medium might comprise a Ti:sapphirecrystal, and the diode laser might have center frequencies on the lowend of the absorption curve of Ti:sapphire, for example blue to violet.Generally the pump beam quality number is M²=1.1 or worse in thehigher-beam-quality dimension and M²=3 or worse in thelower-beam-quality dimension. Generally the M² quality of thelower-beam-quality dimension is at least about three times the M²quality of the higher-beam-quality dimension.

In some embodiments, two diode pump lasers are configured side-by-side,and the optics couple both pump beams into the gain medium in the samedirection. The beams might approach the gain medium side-by-side andoverlap within the gain medium, or they might be spectrally combinedwithin the gain medium. Some configurations include optics tosynchronize a slow axis of the pump beam with a fast axis of the pumpbeam. In general, the spatial overlap of the pump beamhigher-beam-quality dimension and the KLM laser mode is improved by atleast two times over continuous wave mode.

Thus, a Kerr lens modelocked, ultrafast laser includes a pump beamgenerator for generating an anisotropic pump beam, an oscillator cavityincluding a gain medium, and optics for coupling the pump beam into thegain medium and the elements constructed and arranged such that when thelaser is modelocked, spatial overlap of the pump beamhigher-beam-quality dimension and the KLM laser mode is improved, overcontinuous wave mode. Again, a second beam might be added to pump thelaser in various configurations. In some case the laser is direct diodelaser pumped.

A method of inducing stable Kerr lens modelocking in an ultrafast laserincludes the steps of generating one or more anisotropic pump beam, thepump beam having a higher-beam-quality dimension and alower-beam-quality dimension, wherein the M² quality of thelower-beam-quality dimension is at least about three times the M²quality of the higher-beam-quality dimension, constructing and arrangingthe laser elements such that when the laser is modelocked, spatialoverlap of the pump beam higher-beam-quality dimension and the KLM lasermode is improved, over continuous wave mode, Kerr lens modelocking theultrafast laser, and outputting ultrafast pulses.

Finally, a method of inducing stable Kerr lens modelocking in anultrafast laser comprising the steps of generating a two pump beams andfocusing them into a gain medium through the same side of the gainmedium, such that the pump beams overlap in one beam dimensionthroughout the gain medium, constructing and arranging the elements ofthe laser such that when the laser is modelocked, spatial overlap of thepump beams in the overlap dimension and the KLM laser mode is improved,over continuous wave mode, Kerr lens modelocking the ultrafast laser,and outputting ultrafast pulses.

Those skilled in the art of Kerr lens modelocking will appreciate thatmany other embodiments fall within the spirit of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A (Prior Art) shows a Ti:sapphire absorption curve.

FIGS. 1B-1I are diagrams illustrating continuous wave versus Kerr lensmodelocking (KLM) for various configurations of the present invention.

FIG. 2 is a schematic diagram showing a first embodiment of a laser withdirect diode pumping using counter-propagating pump beams.

FIG. 3 is a block diagram showing a second laser embodiment usingsame-side diode lasers and side-by-side pump beams.

FIG. 4 is a block diagram showing a third laser embodiment usingsame-side diode lasers and spectral-combined pump beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The table below provides reference numbers and associate elements forconvenience in discussion the example embodiments of present inventionshown in the figures and discussed below. Note that none of the figuresis to scale.

Ref. No. Element 102 Absorption - Ti-Sapphire 104 Fluorescence -Ti-Sapphire 200 Dual end-pumping embodiment 202 Diode laser 204 Asphericlens 206 Cylindrical telescope 208 Lens 210 Laser cavity 212 Dichroiccurved mirror 214 Dichroic curved mirror 216 Ti-Sapphire crystal 218Output coupler 220 Output 222 Prism 224 Mirror 226 Prism 228 Mirror 300Second embodiment 302 Mirror 304 Mirror 306 Mirror 308 Lens 310 Mirror314 Curved mirror 400 Third embodiment 412 Curved mirror 413 Spectralcombination optics

As used herein, the term “direct diode laser pumping” means diode lasersprovide the pump beams to the cavity directly, via only linear opticalelements such as lenses, mirrors, and/or beam splitters/combiners,without the need for frequency doubling or other nonlinear elements.Note that terms such as vertical and horizontal are used for clarity inunderstanding the figures, but do not limit the configuration of theembodiments.

FIGS. 1B-1I are diagrams illustrating continuous wave versus Kerr lensmodelocking (KLM) for various configurations of the present invention.KLM is only stable if the KLM mode is considerably more efficient thanthe continuous wave (CW) mode. This generally occurs when the spatialoverlap of the pump beam and the KLM laser mode better than the spatialoverlap of the pump beam and the CW mode. The present invention is basedupon the discovery that it is possible to induce stable KLM with ananisotropic beam so long as the spatial overlap of the pump beamhigher-beam-quality dimension and the KLM laser mode better than thespatial overlap of the pump beam and the CW mode. It is not necessarythat the spatial overlap of the higher-beam-quality dimension is good.KLM has been accomplished with beam quality differences of three timesor more between the dimensions.

One important advantage of the present invention is that KLM inTi:sapphire lasers is accomplished with an anisotropic beam such as thatgenerated by currently available 1.0 W, 445 nm diode lasers. The pumpbeams typically have an M² in the x-direction of about 1.3 and an M² inthe y-direction of 3-4—well below the M² of under 1.1 in both dimensiontypically required to accomplish KLM in Ti:sapphire lasers.

FIGS. 1B-1E illustrate both single beam pumping and spectral-combinedbeam pumping configurations. FIGS. 1B and 1C show the spatial overlap ofthe pump beam and the CW cavity mode, for the higher-beam-qualitydimension and the higher-beam-quality dimension, respectively. CW iseasily accomplished, but the overlap is only fair.

FIGS. 1D and 1E show the spatial overlap of the pump beam and the KMLcavity mode, for the higher-beam-quality dimension and thelower-beam-quality dimension, respectively. The overlap between the KLMmode and the higher-beam-quality dimension is substantially better thanthe overlap between the mode and the higher-beam-quality dimension, butthe the overlap between the KLM mode and the lower-beam-qualitydimension is very poor, and far below what was previously thought to besufficient for KLM operation. Yet the KLM power has been found to exceedthe CW power by a factor of 1.5 to 10, producing strong, stable KLMoperation.

Although the absorption of 445 nm pump light is significantly lower thanthe absorption of the 532 nm light typically used in in Ti:sapphirelasers (see FIG. 1, A Prior Art), it works well for the present purpose.The gain region is more uniformly distributed throughout the length ofcrystal 216. In addition, 445 nm light intrinsically focuses to atighter spot than 532 nm light.

Another advantage of the present invention is that it allows two or morediode lasers to be used for pumping, both in counter-propagatingembodiments (see FIG. 2), in side-by-side, non-collinear embodiments(see FIG. 3) and in same-side spectrally combined embodiments (see FIG.4). FIGS. 1F-1I show the spatial overlap of two pump beams and the KMLcavity mode, for the higher-beam-quality dimensions and thelower-beam-quality dimensions, respectively.

Regarding the side-by-side configurations, the inventors have discoveredthat the very asymmetric focal spot shape of the 445 nm pump beam allowsfor considerable flexibility in pumping configurations. For example, asshown in FIG. 3, the beams of two such diode lasers 202 are collimatedand placed side-by-side to enter the lens that focuses the light intoTi:sapphire crystal 216. The beams thus cross in the laser crystal in an“x” in the horizontal direction. This would effectively broaden the gainregion in this x direction—except that this is the dimension where thebeam focal spot size is relatively large anyway. In the verticaldirection, the two beam focal spots can overlap throughout thepropagation through the crystal. FIGS. 1F and 1G illustrate that theoverlap between the KLM mode and the higher-beam-quality dimension issubstantially better than the overlap between the mode and thehigher-beam-quality dimension. FIGS. 1G and 1I show thelower-beam-quality-dimension crossing, meaning the overlap is even worsefor this dimension. Stable KLM operation is still achieved because ofthe excellent overlap between the KLM mode and the higher-beam-qualitydimension.

Thus, embodiments of the present invention take advantage of theasymmetric focal spot to allow two pump laser beams to enterside-by-side from the same side of the crystal. This is new. To theinventors' knowledge, no KLM lasers have successfully employed two pumpbeams entering from the same side of the crystal. This allows using morethan one pump laser in a way that avoids light from one pump diode fromentering into the other.

FIG. 2 is a schematic diagram showing a first embodiment of a laser 200with direct diode pumping using dual-end pumping. Ti:sapphire lasers arewell understood in the field of ultrafast lasers, and in most respects,the following embodiments are conventional Ti:sapphire lasers, asdescribed (for example) in the seminal paper “Generation of 11-fs pulsesfrom a self-mode-locked Ti-Saphire laser”, Asaki et al., Optics Letters,Jun. 15, 1993, Vol. 18, No. 12. A currently available example of such alaser is the KMLabs Griffin™. Embodiments of the present invention havebeen modified to run at low pumping thresholds by using slightly shorterfocal length curved mirrors 212, 214 (86 mm radius of curvature ratherthan the more typical 100 mm radius of curvature) as well as a 1% outputcoupling mirror 218 centered at 800 nm.

The laser 200 of FIG. 2 is pumped by two 1.2 W, 445 nm laser diodes 202,each of which is collimated by a 6 mm aspheric lens 204. Diode lasers202 have an emitter area of approximately 15 um×1 um. Diode lasers 202are polarized along the long axis of the diode output facet. For bothdiodes, this long facet axis is oriented in the horizontal (x)direction, such that the beam diverges faster in the vertical (y)direction. To avoid pushing the diode lasers to their rated limit, thelaser diodes are operated at 1 W each, with 1.8 W delivered through alloptics to crystal 216.

In this embodiment, each diode laser 202 is collimated by an asphericlens 204 followed by a 3:1 cylindrical telescope 206 applied to the slowaxis. In this example, the fast and slow axes of the diode beam, whencollimated only with the aspheric lens 204, focus to different planes,so the cylindrical telescope 206 is adjusted to place the two beamwaists at the same plane and to decrease the spot size of the slow axisbeam dimension in the crystal.

The pump beams are linearly polarized, and thermoelectrically (TEC)cooled (not shown) to control the center wavelength and to stabilize theoutput power. As an alternative, other methods of cooling such as wateror cryogenic cooling may be used, or cooling may not be necessary.

The pump beams are focused with 50 mm lenses 208 onto Ti:sapphirecrystal 216 from both sides (“dual end-pumping”). Crystal 216 is a 4.75mm thick Brewster angle Ti:sapphire crystal with an absorptioncoefficient of 2 cm-1 at 445 nm, giving 60% absorption of the pump lightin this example.

The intracavity prisms 222, 226 for dispersion compensation are fusedsilica, and are 650 mm apart (path length). The output coupler 218 is 1%at 800 nm. Detuning of the pump light wavelengths helps reduce feedbackfrom one diode into the other.

The oscillator 210 uses a pair of 86 mm radius of curvature (ROC)dichroic mirrors 212, 214 surrounding a Brewster angle 4.75 mm thickTi:sapphire crystal 216 with an absorption coefficient of 2 cm⁻¹ at 445nm, giving 60% absorption of the pump light. The crystal has a figure ofmerit greater than 200. KLM may be initiated in a number of ways, suchas by rapidly moving on the prisms in order to generate an intensityspike.

This embodiment has been shown to achieve stable Kerr lens modelockingwith a 21 nm spectrum, and an output power of 44 mW. Pulses with 15 fsduration were also achieved. The CW power is lower than the KLM power bya factor of three, indicating a strong preference for KLM and highlong-term stability.

FIG. 3 and FIG. 4 are embodiments of the present invention usingcombined same-side pump beams from diode lasers 202.

FIG. 3 is a block diagram showing a laser embodiment using same-sidepump beams, wherein the pump beams enter crystal 216 side-by-side. Thisis accomplished using mirrors 302, 304, 306 as shown in the figure. Lens308 focuses the side-by-side beams into crystal 216, where they, incombination cause KLM. Referring back to FIGS. 1H and 1I is useful invisualizing the KLM operation of this embodiment. The beams are notprecisely collinear, but overlap well in the higher-beam-qualitydimension.

FIG. 4 is a block diagram showing a third laser embodiment usingmultiple same-side pump diode lasers 202 and spatially-overlappedspectrally-combined pump beams. This is accomplished with the use ofspectral combination optics 413A, 413B, etc. Diode lasers 202A, 202B,202C, etc. are selected to generate pump beams with slightly differentcenter frequencies. This allows the pump beam from laser 202A to passthrough spectral combination optic 413A, while the pump beam from diodelaser 202B reflects off of spectral combination optic 413A. Similarly,the pump beams from diode lasers 202A and 202B pass through spectralcombination optic 413B, while the pump beam from diode laser 202Creflect off of spectral combination optic 413B to combine with the pumpbeams from diode lasers 202A and 202B. Telescope lens 206 adjusts thefast and slow axes of all three pump beams, and lens 208 focuses theminto the laser cavity 210, which operates as previously shown togenerate output beam 220. The present inventors have accomplished thecombination of three pump beams so far, and those skilled in the art ofultrafast lasers will appreciate how the design is modified to combinemore pump beams. This embodiment of the invention has demonstrated morethan 300 mW modelocked output power and output spectra of more than 140nm full-width-half-maximum, which supports pulse durations of less than10 fs.

While the exemplary preferred embodiments of the present invention aredescribed herein with particularity, those skilled in the art willappreciate various changes, additions, and applications other than thosespecifically mentioned, which are within the spirit of this invention.For example, while the present invention is particularly useful inaccomplishing Kerr lens modelocking with blue light diode lasers inTi:sappire crystals, the technique is also useful in inducing KLM inother oscillators and/or with other anisotropic pump beam sources.

What is claimed is:
 1. A direct laser diode pumped, Kerr lensmodelocked, laser comprising: an oscillator cavity including a gainmedium; a diode pump laser that generates an anisotropic pump beam,wherein the pump beam has a higher-beam-quality dimension and alower-beam-quality dimension, and wherein the pump beam wavelength fallswithin the absorption band of the gain medium; and optics for directlycoupling the pump beam into the gain medium; the elements constructedand arranged such that when the laser is modelocked, spatial overlap ofthe pump beam higher-beam-quality dimension and the KLM laser mode isimproved, over spatial overlap of the pump beam higher-beam-qualitydimension and the continuous wave mode.
 2. The laser of claim 1 whereinthe pump beam wavelength is within the full width at half maximum of theabsorption curve of the gain medium.
 3. The laser of claim 2 wherein theM² quality of the lower-beam-quality dimension is at least about threetimes the M² quality of the higher-beam-quality dimension.
 4. The laserof claim 1 wherein the pump beam wavelength is below the full width athalf maximum of the absorption curve of the gain medium.
 5. The laser ofclaim 1 wherein the pump beam wavelength is above the full width at halfmaximum of the absorption curve of the gain medium.
 6. The laser ofclaim 1 wherein the gain medium is a Ti:sapphire crystal.
 7. The laserof claim 6 wherein the pump beam wavelength is within the full width athalf maximum of the absorption curve of the Ti:sapphire crystal.
 8. Thelaser of claim 7 wherein the M² quality of the lower-beam-qualitydimension is at least about three times the M² quality of thehigher-beam-quality dimension.
 9. The laser of claim 6 wherein the pumpbeam wavelength is below the full width at half maximum of theabsorption curve of the Ti:sapphire crystal.
 10. The laser of claim 6wherein the pump beam wavelength is above the full width at half maximumof the absorption curve of the Ti:sapphire crystal.
 11. A Kerr lensmodelocked, ultrafast laser comprising: an oscillator cavity including again medium; and a pump beam generator for generating an anisotropicpump beam, the pump beam having a higher-beam-quality dimension and alower-beam-quality dimension, the pump beam wavelength falling withinthe absorption band of the gain medium; optics for coupling the pumpbeam into the gain medium; the elements constructed and arranged suchthat when the laser is modelocked, spatial overlap of the pump beamhigher-beam-quality dimension and the KLM laser mode is improved, overcontinuous wave mode.
 12. The laser of claim 11 wherein the pump beamwavelength is within the full width at half maximum of the absorptioncurve of the gain medium.
 13. The method of inducing stable Kerr lensmodelocking in an ultrafast laser comprising the steps of: (a)generating an anisotropic pump beam, the pump beam having ahigher-beam-quality dimension and a lower-beam-quality dimension,wherein the M² quality of the lower-beam-quality dimension is at leastabout three times the M² quality of the higher-beam-quality dimension,the pump beam wavelength falling within an absorption band of a gainmedium; (b) coupling the pump beam into a gain medium of the laser; (c)constructing and arranging the laser elements such that when the laseris modelocked, spatial overlap of the pump beam higher-beam-qualitydimension and the KLM laser mode is improved, over continuous wave mode;(d) Kerr lens modelocking the ultrafast laser; and (e) outputtingultrafast pulses.
 14. The method of claim 13 wherein the step ofgenerating an anisotropic pump beam generates a pump beam having awavelength within the full width at half maximum of the absorption curveof the gain medium.
 15. The method of claim 14, wherein step (a)generates two anisotropic beams, and wherein step (b) couples both beamsinto the gain medium.